This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The regulatory protein MphR(A) has recently seen extensive use in Synthetic Biological applications such as metabolite sensing and exogenous control of gene expression. This protein negatively regulates the expression of a macrolide 2'-phosphotransferase I resistance gene (mphA) via binding to a 35bp DNA operator upstream of the start codon, and is de-repressed by the presence of erythromycin. Our goal in this project was to determine the difference in structure between the erythromycin bound and free Mphr(A) in order to understand how it might be engineered to respond to different macrolide antibiotics.